1. Field of the Invention
The present invention relates to a zoom lens and an image pickup apparatus equipped with the same. The present invention can suitably be applied to broadcast television cameras, video cameras, digital still cameras, and film cameras.
2. Description of the Related Art
In recent years, zoom lenses having a high zoom ratio, a wide angle of view, and high optical performance have been demanded for use in image pickup apparatuses such as television cameras, film cameras, digital cameras, and video cameras. As zoom lenses having a high zoom ratio and a wide angle of view, positive-lead type, four-unit zoom lenses that have four lens units in total including a lens unit having a positive refractive power disposed closest to the object side have been known. A known four-unit zoom lens of this type comprises, in order from the object side to the image side, a first lens unit for focusing, a second lens unit having a negative refractive power for zooming, a third lens unit having a negative refractive power for correcting image plane variations during zooming, and a fourth lens unit having a positive refractive power for imaging. Four-unit zoom lenses of this type having a high zoom ratio and a wide angle of view have been known from U.S. Pat. Nos. 6,124,982, 5,966,246, Japanese Patent Application Laid-Open No. H09-258102, and U.S. Pat. No. 6,545,818. These patent documents disclose zoom lenses having a zoom ratio of approximately 10 to 12, an angle of view of approximately 80° to 100° at the wide angle end.
The positive-lead type four-unit zoom lens can relatively easily designed to have a high zoom ratio and a wide angle of view. However, if it has a zoom ratio as high as 10 and an angle of view as wide as 80° or wider, large variations of aberrations will occur during zooming, making it difficult to achieve high optical performance throughout the entire zoom range. In particular at zoom positions near the telephoto end, large magnification chromatic aberration and on-axis chromatic aberration occur. Therefore, in order to achieve performance that provides high quality images, it is important to achieve good correction of chromatic aberration in terms of not only first-order spectrum but also second-order spectrum. In order to achieve good correction of chromatic aberration and high optical performance while achieving a high zoom ratio of approximately 10 and a wide angle of view of 80° or wider in the four-unit zoom lens of this type, appropriate lens configuration of the first lens unit is an important factor. Inappropriate lens configuration of the first lens unit lead to large variations in aberrations such as chromatic aberration, spherical aberration, halo, and coma, making it difficult to achieve high performance with a high zoom ratio and a wide angle of view. In particular, when a zoom lens is designed to have a high zoom ratio of approximately 10 and a wide angle of view of approximately 100° at the wide angle end, large on-axis chromatic aberration tends to occur at the telephoto end. Furthermore, large residual aberrations with respect to second-order spectrum tend to remain. It is difficult to correct these aberrations satisfactorily.